There is a wicked genius to the term “person of interest,” used to describe someone in the vicinity of a crime who falls just short of official suspicion. There is no careful or innocent way to use this term, and unlike “suspect” or “material witness,” it has no legal definition. Instead, it is a damning shade of gray, chilling yet clumsy, and shrapnel-like in its effect. One is not quietly deemed a “person of interest,” as the men who have been linked with the 2001 anthrax attacks and the Atlanta Olympic bombings could attest, and one does not shed the tag easily.

It is also the kind of swift, nuanced statement that Professor Lee — the sort-of-accused in Susan Choi’s engrossing third novel “A Person of Interest” (Viking, 356 pages, $24.95) — is incapable of. Lee — we never learn his first name — is a profoundly awkward, middle-age mathematician whose self-perception rarely accords with how others understand him. As a young graduate student, he earns the honorific “princely” (or “Prince Lee”) though he is utterly lacking in social grace. Moments of nervous insecurity are read as inscrutable suaveness, while indignation or shock comes across as tongue-tied bumbling. And so he avoids it all, sitting alone, comfortably numb, in a barely-furnished house at a mid-tier university, drinking his cans of beer in silence.

Lee’s misanthropy would be fine if his only obligation was to himself. But he is unwittingly absorbed into a national tragedy when a letter bomb explodes in the office of Rick Hendley, his younger and infinitely more popular colleague. The blast on the other side of their shared wall rips open a “raw, never-mined vein” of dislike Lee held for Hendley: Lee doesn’t feel all too bad about what has happened. It’s a disturbing, of-the-moment lucidity we assume will soon pass, even as the bewildered Lee somehow manages to string together a heroic, eloquent condemnation of the bomber on the local news.

But as the community spills forth with the rote empathy these episodes require, Lee feels nothing more worth sharing. His heart will not permit him to view Hendley as the martyr everyone else believes him to be. As mourning migrates from private ritual to public gesture, Lee is stained by his inability to summon (or at the very least perform) something resembling grief. Everyone around him eagerly revisits the tragedy and the feelings it inspired, feasting on their proximity to true history. Instead, Lee comes upon the petty, self-absorbed ugliness of his true inner self. The impossibility of retreating to his anonymous and comfortable existence — the life of a “short poppy,” he and his lone friend joke — nearly destroys him. When Lee receives a series of cryptic letters, he begins to suspect that the bomb, as well as others targeting notable scientists, is the work of a former friend from graduate school. He grows obsessed with proving that the “Brain Bomber,” as the media calls him, is Lewis Gaither, a mediocre mathematician and religious zealot whose wife Lee stole decades earlier.

But Lee’s impassiveness ensnares him in a web of hurt feelings and official mistrust. At every turn he is unable to explain himself, to prove that Gaither exists, to justify his absence from a campus memorial for Hendley. Colleagues and neighbors murmur about his loner ways; investigators wonder if his off-putting placidity conceals something worrisome. Unable to convince anyone of his innocence, he too becomes a “person of interest.” And unable to withstand the burden that term brings, he flees in pursuit of the mysterious letter-writer.

The novel’s pace quickens once Lee embarks on this bizarre search for the Brain Bomber, a fictionalized Theodore Kaczynski. But the story of the Unabomber is richer than most writers’ imaginations, and this is where Ms. Choi’s novel is infected with an unwelcome tidiness.

Ms. Choi is masterful when describing Lee, who loudly, mournfully echoes another real figure: Wen Ho Lee, the Taiwanese-born American scientist wrongfully accused of passing nuclear secrets on to China. The fictionalized Lee is connected to a far smaller crime, and he is puerile and graceless where the actual Lee was polite and dignified. Yet this only underscores the nightmare that “A Person of Interest” describes. This Lee feels guilt for his unkind thoughts; there is no high road available to him. It is agonizing to witness Lee’s neuroses muddle the words issuing from his mouth. He is unable to say the right thing or set people at ease, and on the rare occasion he chooses his words wisely, his tone — his true feelings on the matter — betrays him.

In this way, Lee comes across as more Everyman than enigma. We humor the uglier aspects of ourselves, the sliver of our minds that might agree with what the Brain Bomber has to say. But we accept that we will never be our “idealized self,” and we live with “the self’s acceptance of the things it can’t be.” For most, this simply means pursuing meaning elsewhere: spirituality, as with his former colleague Gaither; eco-activism, as with Lee’s estranged daughter, or even terrorism, as with the Brain Bomber. But what of Lee? Possessing nothing but his job and a secondhand briefcase, he seems suspicious — if not for his terrorist potential, then for his rejection of community.

As with Ms. Choi’s last novel, the Pulitzer Prize finalist “American Woman” (2003), “A Person of Interest” fixates on the fate of those who stand off to the side, only to one day find themselves swept away, against their wishes, in the great gusts of history. At times this feels like a sly metaphor for Lee’s oft-mentioned, never-unpacked identity: Is Lee unreadable because he is, as some call him, “Oriental”? Why does everyone assume he is Chinese or Japanese? (There is a faint hint that he is Korean.) And what of the investigator who mentioned a rumor that Asians — inscrutable, passive, emotionless — were immune to lie detector tests?

There is very little dialogue in “A Person of Interest” — Lee, like most neurotic people, spends most of his time fretting over what to say. Instead, we spend most of the novel’s best moments inside his cluttered head, as he weathers multi-entendre accusations, veers toward mania, and hears his own voice “scaling, ascending, barely corresponding any longer to an inner condition.” It is as though his voice is transmitting from somewhere else, as though he is witnessing someone else’s fate, as though this isn’t happening to him, or to you.

Mr. Hsu teaches English at Vassar College. He last wrote for these pages on Bernard Schlink.